Successful field demonstration of shifted excitation Raman difference spectroscopy for soil analysis

Soil nutrient management is becoming increasingly important to improve crop productivity, increase soil health, and reduce excessive use of commercial fertilizers. In situ sensor techniques have the potential to provide necessary information for an on-site decision management system and therefore investigated at the Ferdinand-Braun-Institut. Essential soil nutrients are composed of elements like phosphorus (P), nitrogen (N) and many more. To examine the composition of soil, non-destructive sensors are needed that are able to distinguish not only the elements but to determine chemical compounds and provide important information regarding, e.g., the spatial distribution of plant available nutrients.

Raman spectroscopy is a powerful and well-established spectroscopic technique which provides Raman signals in a fingerprint-like spectrum. This way, molecules can be identified, which enables qualitative and quantitative investigations of samples. However, in outdoor applications the weak Raman signals are often masked by strong fluorescence background signals and daylight interferences. Shifted excitation Raman difference spectroscopy (SERDS) efficiently separates Raman signals from disturbing background signals using a physical approach with two slightly shifted laser wavelengths. The Ferdinand-Braun-Institut has already performed Raman and SERDS field experiments [1] in an apple orchard and presented SERDS as a promising tool for soil analysis [2].

Beside background interferences, soil shows a strong heterogenous distribution of substances. Consequently, the FBH has selected a customized Raman probe for soil analysis. Beside other requirements, a sufficiently large excitation and collection spot size at the sample had to be generated, which provides representative information on soil components. To enable on-site measurements, the Ferdinand-Braun-Institut has developed a portable SERDS sensor system for soil analysis within the research project RaMBo (Raman-Messsystem zur ortsspezifischen Bodenanalytik) as part of the I4S (Intelligence for Soil) consortium. Fig. 1 shows the realized portable SERDS system applied in a first field experiment successfully performed on experimental plots in Marquardt – a research site of our project partner Leibniz Institute for Agricultural Engineering and Bioeconomy (ATB).
An in-house developed one-chip dual-wavelength 785 nm diode laser is used as excitation light source for SERDS as core element of the system. The diode laser is integrated into an FBH turnkey system to control and transfer the laser light via an optical fiber to the Raman probe. Fig. 2 shows the customized probe tip for soil investigations. The backscattered Raman signals are then launched into a compact spectrometer for wavelength-selective detection. All elements including battery are integrated into a rugged case suitable for field operation. An in-house developed software is responsible for control and data management.

First results of the field test are promising – data evaluation of the situ SERDS experiments is currently in progress. The results gained from SERDS measurements will then be compared with complementary sensor techniques to combine sensory information for an improved on-site decision soil management system.

This study was funded by the Federal Ministry of Education and Research (BMBF) under contract 031A564C through the funding measure BonaRes (Soil as a Sustainable Resource for the Bioeconomy) within the consortium I4S (Intelligence for Soil).

Publications

[1] M. Maiwald, A. Müller, B. Sumpf, G. Tränkle, “A portable shifted excitation Raman difference spectroscopy system: device and field demonstration”, Journal of Raman Spectroscopy, vol. 47, no. 10, pp. 1180-1184 (2016).

[2] L. S. Theurer, M. Maiwald, B. Sumpf, “Shifted excitation Raman difference spectroscopy: A promising tool for the investigation of soil”, European Journal of Soil Science, vol. 72, no. 1, pp. 120-124 (2021).